I'm not sure I fully understand your question but here are my main set ups: I run a 40g air pump split to my 2 10g tanks that each have a 15g sponge filter, a 20g air pump on another 10g also with a 15g sponge filter, a 20g on a 5g with a 10g sponge filter. I don't think the air pump "size" really makes a difference, but I could be wrong, I just don't see any difference with my tanks.

Oh, well IMO that is a personal choice & of course depends on the stocking. Lots of people prefer to over filter. I run one filter rated for more than the tanks capacity rather than having two filters on one tank, I don't like the look of multiple fitlers. I'd rather the fish have that extra little bit space.

The thing Is I hadnt planned to over filter this tank or even to convert it but the filter I planned to use for it is going out. I prefer to over filter as well so I will look at my options when I get paid

I dont think you can ever have too much filtration. I use multiple sponge filters on many tanks and it is also nice to have an extra seasoned sponge filter if you ever want to jumpstart another tank. Go for it.

Think of filtration in this way, can our air ever be to clean to breathe? It is just not possible to have water that is too clean. Remember, that water is not only what your fish breathes, it's also his dinner table and bathroom. Does that make a little more sense?

Lol I know all of that but I also know Betta are sensitive to water movement with those fins so usually its recommended not to overfilter too much. However I was more wondering since its a sponge filter if its fine to do.